#include <iostream>
#include <string>

std::string s, t;

int main()
{
	std::cin >> s >> t;
	int l = s.size();
	std::string ans = s;
	ans[l - 1]++;
	for (int j = l - 1; j; j--)
		if (ans[j] > 'z') ans[j] = 'a', ans[j - 1]++;
	if (ans[0] > 'z' || ans >= t)
		std::cout << "No such string\n";
	else
		std::cout << ans << '\n';
}

